1. Field of the Invention
The present invention relates to a surface emitting laser.
2. Description of the Related Art
Conventionally, the light intensity of a semiconductor laser is controlled to a desired level by monitoring the light intensity with a photodetector (PD), and increasing or reducing the current driving the semiconductor laser according to the monitoring result. In an edge emitting laser, light is emitted from two facets thereof. Light from one facet is combined with a lens or a fiber and is used as output light; and light from the other facet enters the photodetector to monitor the light amount.
Unlike the edge emitting lasers, the vertical-cavity surface emitting laser (VCSEL) emits light in a direction perpendicular to the surface of the substrate. See, Japanese Patent Laid-Open No. 07-335976. VCSELs can be easily arranged in a two-dimensional array at a higher density than the edge emitting lasers. By using a high density VCSEL array for electronic photography, high resolution and high speed photography can be achieved.
A general VCSEL, which includes an upper and a lower distributed Bragg reflector (DBR) with a resonator therebetween, emits light in a direction perpendicular to the surface of the substrate. If the amount of light from the VCSEL structure is to be monitored by extracting laser light from the opposite side to the light emitting side, as in the edge emitting lasers, light must be extracted through the substrate to the rear side of the substrate.
Another type of semiconductor laser having a structure different from the conventional VCSEL is proposed by Japanese Patent Laid-Open No. 2000-332351. The type of semiconductor laser proposed by Japanese Patent Laid-Open No. 2000-332351 vertically emits light with a photonic crystal (PhC). Such a laser will be referred to as a PhC surface emitting laser. A PhC surface emitting laser resonates in the horizontal direction, but emits light in the direction perpendicular to the surface of the substrate because of PhC diffraction. The PhC surface emitting laser is different from a VCSEL in structure, but is similar in emitting light in the direction perpendicular to the surface of the substrate. Accordingly, for monitoring light from the PhC surface emitting laser at the substrate side, the light must also be extracted to the rear side of the substrate through the substrate.
Semiconductors absorb light (photons) having a higher energy than their band gaps (by interband transition). For example, GaAs absorbs light having wavelengths of less than 880 nm. The absorption coefficient of this absorption by interband transition is ten times or more larger than that of free electron absorption or intervalence band absorption. Accordingly, the substrate cannot transmit light to the rear surface thereof. Therefore, for example, in surface emitting lasers of 850 nm, 780 nm, or 660 nm formed on a GaAs substrate, light is highly absorbed to the substrate by interband transition in the substrate material, and the light cannot be easily monitored at the substrate side even if the DBR or the like at the substrate side is adjusted to reduce the reflectance.